This invention relates to a high frequency receiving circuit for use in a mobile communication apparatus, and more particularly relates to a high frequency receiving circuit for use in a mobile telephone employing the CDDM (Code Division Multiple Access) system, for instance a cellular mobile telephone (IS95A Spec.; 800 MHz) and a portable or pocketable mobile telephone (PN-3385 Spec.; 1.9 GHz).
Heretofore, as show in FIG. 8, the high frequency receiving circuit of this kind is provided with an antenna (ANT) 51, a duplexer (DUP) 52, an amplifier with low noise (LNA) 53, an interstage band-pass filter (IS-BPF) 54, a mixer (MIX) 55, an intermediate frequency band-pass filter (IF-BPF) 56, a local amplifier (LCA) 57, a voltage control oscillator (VCO) 58, and a phase locked loop circuit (PLL) 59.
A receiving band-pass filter (R-BPF) 52a of the duplexer 52 and the interstage band-pass filter 54 are band-pass filters commonly having a passing band equal to the receiving band of a station's own. These band-pass filters act to prevent the transmission wave from their own station and the unnecessary receiving wave coming thereto from being inputted to the low noise amplifier 53 and the mixer 55, thereby checking deterioration in the receiving sensitivity and occurrence of the interference. Main characteristics required to both of the receiving band-pass filter 52a and the interstage band-pass fillter 54, are an insertion loss characteristic within the passing band and an attenuation characteristic outside the passing band. These characteristics will be determined totally taking account of the specification of the receiver.
The intermediate band-pass filter 56 is a band-pass filter which determines a channel band characteristic. The characteristic required to this band-pass filter 56 is to keep a constant phase within the passing band and to have the high attenuation characteristic outside the same. In order to realize such a characteristic, there is used a band-pass filter of the interdigital system. The center frequency of the band-pass filter of the interdigital system is generally set to a value of 70 through 110 MHz in view of its performance and price as well.
For instance, in the high frequency receiving circuit of the single conversion type shown in FIG. 8, the center frequency of the intermediate frequency band-pass filter 56 becomes the intermediate frequency, of which value is set to a value between 70 and 110 MHz. In case of the circuit in FIG. 8, the intermediate frequency is set to a value of 85.38 MHz.
However, the prior art high frequency receiving circuit as mentioned above, has often experienced such a problem that when an interference wave with a specific frequency is intermixed with the receiving wave, an interference that is so-called single-signal interference and/or intermodulation interference is caused in the range of the intermediate frequency.
The single-signal interference is a phenomenon which occurs when an interference wave with a frequency (fu) is intermixed with a receiving wave to be inputted to the mixer 55. The frequency (fu) is expressed as follows:
______________________________________ fu = fch .+-. (1/2)fi ((+) when fch &lt; fL and (-)when fch &gt; fL) where fch: receiving frequency (the center frequency of a selected channel), fi: intermediate frequency, and fL: local oscillation frequency of the voltage control oscillator 58. ______________________________________
Such interference wave is mixed with the local carrier wave in the mixer 55 to generate an interference signal in the range of the intermediate frequency.
On the other hand, the intermodulation interference is a phenomenon which takes place when a plurality of interference waves having a frequency apart from the receiving frequency fch of the selected channel at an equal interval, for instance two interference waves having frequencies of fch+1 MHz and fch+2 MHz respectively, are intermixed with the receiving wave to be inputted to the mixer 55. Such interference waves cause interference signals in the position of the receiving frequency or the intermediate frequency owing to intermodulation in the low noise amplifier and the mixer 55.
A lot of proposals have been made so far in order to prevent generation of such interference signals. However, according to the PN-3385 standard, the receiving band width is widely set to about 60 MHz while the intermediate frequency is set on such a lower side as 85.38 MHz, and the frequency of the above interference wave resides within the receiving band, so that it is difficult to attenuate the interference wave by means of the receiving band-pass filter 52a or the interstage band-pass filter 54. As to the endurance characteristic against interference, a very high level is requested according to the PN-3385, for instance 71 dB as the endurance quantity against the single-signal interference and 58 dB as the endurance quantity against the intermodulation interference. Thus, such technical requirement could hardly be satisfied for the same reason. The endurance quantity against the interference is defined as a level of the interference signal which causes deterioration in the receiving sensitivity. For instance, the endurance quantity against the single-signal interference being 71 dB means that the receiving sensitivity starts its deterioration if the single-signal interference wave, of which the level is at least 71 dB higher than the level of a desired signal (i.e. signal sent by a caller side and to be received by an addressed receiver) is intermixed through the antenna 1.
As a countermeasure for improving the endurance characteristic against interference, it might be considered to use a double balanced mixer 55 for suppressing generation of the interference signal. However, this would not work to adequately satisfy the above-mentioned endurance characteristic against interference.
Also, as a way for improving the endurance characteristic against the single-signal interference that, as shown in FIG. 9, it might be proposed to provide a high frequency receiving circuit of the double conversion type, in which the center frequency of the passing band of the 1st intermediate frequency band-pass filter (1st IF-BPF) 61 is increased to 210.38 MHz while the same of the 2nd intermediate frequency band-pass filter (2nd IF-BPF) 63 is set to 85.38 MHz. In the circuit arrangement like this, the interference wave frequency fu comes to be outside the receiving band, and the interference wave with the frequency fu is made to attenuate between the receiving band-pass filter 52a and the interstage band-pass filter 54 while the interference wave existing in the receiving band is made to attenuate between the 1st intermediate band-pass filter 61 and the 2nd intermediate frequency band-pass filter 63. As for the endurance quantity against the interference, however, in order to fully satisfy the above-mentioned standard by relying on the circuit as arranged above, the 2nd mixer (2nd MIX) 62 would require another countermeasure against the interference wave, for instance making it be of the double balance type. Consequently, this would result in presentation of additional problems such as increase in the number of circuit constituents and in the operational current consumption.
Accordingly, an object of the invention is to provide a novel and improved high frequency receiving circuit for use in a mobile communication apparatus having an excellent endurance characteristic against interference, by which the influence of the interference wave having a predetermine frequency can be minimized even if its intermixing happens.
Another object of the invention is to provide a novel and improved high frequency receiving circuit for use in a mobile communication apparatus which has an excellent endurance characteristic against the single-signal interference as well as against the intermodulation interference, and meets various strict standards, especially the IS95A Standard and the PN-3385 Standard.
Still another object of the invention is provide a novel and improved high frequency receiving circuit for use in a mobile communication apparatus which may be simply designed without modifying the prior art high frequency receiving circuit to a great extent, thus being capable of manufacturing it at a low cost.
Still another object of the invention is provide a novel and improved communication apparatus, of which the endurance characteristic against interference may be improved without increasing the operational current consumption comparing with the prior art high frequency receiving circuit, thus being preferably and suitably applicable to a portable telephone requiring less power consumption.